1. Field of the Invention
The invention relates generally to GPS receivers and, more particularly, to GPS receivers that incorporate inertial systems.
2. Background Information
A global positioning system (GPS) receiver requires signals from a number of GPS satellites in order to accurately calculate its position. The GPS receiver acquires and tracks signals consisting of carrier, pseudo random codes and modulated data from various satellites. The receiver correlates locally-generated codes with the codes received from the respective satellites to derive timing information relating the receiver time relative to the local generation of code chips to the satellite time at the times of the transmission of the corresponding code chips. The timing relationship between the receiver time and the transmission times of the various signals at the various satellites can be used in conjunction with the modulated data from the various satellites to generate a position of the receiver with respect to a reference frame shared with the satellites, for example, the earth centered earth fixed (ECEF) frame.
At least 4 satellites are required to provide a GPS position solution. Corrections to pseudo random measurements with respect to a fixed position, which is generated at a “base station” receiver, can be used to derive a relative position of the local receiver with respect to the base station position. Carrier measurements taken at the base station and the local receiver can be mixed in a known manner to generate precise relative position measurements, provided sufficient signals are available either instantaneously or over time to make it possible to resolve associated initial ambiguities of the carrier measurements. Specifically, the ambiguities can be used in conjunction with the current carrier measurements to generate the differences in geometric distances between the local receiver, the base station receiver and the various satellites whose signals are, at the time, being tracked by the two receivers.
The ambiguity resolution process typically takes between 30 seconds and 1 minute. The time the process takes is dependent on the number of satellites tracked with the process taking longer if only a relatively small number of satellites are being tracked.
If the GPS receiver does not, at any given time, receive the signals from a minimum number of satellites, the GPS receiver is unable to calculate position information. Thereafter, when the satellite signals are again available to the GPS receiver, that is, the satellites are “visible” to the receiver, the receiver must re-acquire the signals before the receiver can resume its position calculations. Signal re-acquisition involves re-synchronizing locally-generated codes to the codes in the received signals, in order to again track the signals. Following the signal re-acquisition process, the receiver must, when operating in precise differential mode, re-resolve the carrier ambiguities before precise positions become available.
During the re-acquisition operations, the user is without navigation information, and during the resolution process the user is without precise position information. Accordingly, the speed with which the receiver re-acquires the signals and resolves the carrier ambiguities is of paramount importance to the user.
The GPS satellites may become unavailable to the GPS receiver for various periods of time in, for example, urban environments, when the GPS receiver travels under a bridge, through a tunnel, or through what is referred to in the literature as an “urban canyon,” in which buildings block the signals or produce excessively large multipath signals that make the satellite signals detrimental to use for position calculations. In addition, other environments, such as racetracks that include grandstands or high fences, may similarly block the signals and produce large multipath signals. Thus, operating the GPS receiver while passing through natural canyons and/or on race tracks or other areas in which satellite coverage is sparse, and so forth, may similarly result in the receiver being unable to track a sufficient number of satellites. Thus, in certain environments the navigation information may be available only sporadically, and GPS-based navigation systems may not be appropriate for use as a navigation tool.
One solution to the problem of interrupted navigation information is to use an inertial system to fill-in whenever the GPS receiver cannot observe a sufficient number of satellites. The inertial system has well known problems, such as the derivation of the initial system (position, velocity and attitude) errors as well as IMU sensor errors that tend to introduce drifts into the inertial position information over time. Accordingly, a system that uses GPS position information to limit the adverse effects of the drift errors on the position calculations in the inertial system has been developed. Further, such as system provides inertial position and velocity information to the GPS system, to aid in signal re-acquisition and in the process to resolve ambiguities. Such a system is described in U.S. Pat. No. 6,721,657 which is incorporated herein in its entirety by reference.
The patented combined GPS and INS system in real time combines the information from GPS and inertial sub-systems to aid in signal re-acquisition and in the resolution of associated carrier ambiguities. The INS/GPS receiver thus provides accurate and uninterrupted navigation information in an environment in which sufficient numbers of GPS satellites are not continuously in view. The combined system has been improved by modifying the INS Kalman filter to include GPS and/or other observables, i.e., measurements, that span previous and current times. The INS Kalman filter utilizes the observables to update position information relating to previous and current positions and propagate current position, velocity and attitude related information. The INS/GPS receiver thus produces even more accurate estimates of inertial position and velocity in the environment in which sufficient numbers of GPS satellites are not continuously in view. The improved system is described in co-pending patent application entitled INERTIAL GPS NAVIGATION SYSTEM WITH MODIFIED KALMAN FILTER Ser. No. 10/758,363 filed Jan. 15, 2004.
The INS/GPS systems described above work well in environments in which the INS system alignment can be established when the receiver is stationary. The alignment of the INS system is often derived with a combination of the accelerations as measured by the INS system and compared with the gravity vector, and the angular rate measured by the INS system and compared to the earth rate. The computation requires that the INS system not experience any specific forces except gravity and not be physically rotating except from earth rotation. Further, the computation requires that the gyro bias in the unit is small compared to the earth rate.
In environments in which the receiver is not stationary at start-up, for example, in a race car that rolls out of a garage (no GPS) and onto a race track, the INS system alignment must be obtained when the receiver is moving at a relatively high rate of speed and around corners. In such environments the INS system senses significant non-vertical specific forces and motion induced rotation. Accordingly, the standard method for alignment could, under these circumstances, easily give roll and pitch errors of 45 degrees or more. This type of initial error leads to non-linear errors in the Kalman filter estimators, which causes the filter to take a prohibitively long time to estimate its system errors well enough to make the system useful.